1. Field of the Invention
The invention relates to an electroluminescent display device, particularly to an electroluminescent display device having color filter layers.
2. Description of the Related Art
An organic electroluminescent (hereafter, referred to as EL) element is a self-emission element. An organic EL display device using the organic EL element is receiving an attention as a new display device substituted for a CRT or an LCD.
FIG. 5 is a schematic cross-sectional view showing a pixel of a full-color organic EL display device of the conventional art. A numeral 200 designates a glass substrate, a numeral 201 designates an organic EL element driving TFT (thin film transistor) formed on the glass substrate 200, and a numeral 202 designates a first planarization insulating film. A numeral 203 designates an anode layer made of ITO (indium tin oxide) which is connected with the TFT 201 and extends over the first planarization insulating film 202, and a numeral 204 designates a second planarization insulating film formed so as to cover an end portion of the anode layer 203. A numeral 205 designates R (red), G (green), and B (blue) organic EL layers formed on the anode layer 203, and a numeral 206 designates a cathode layer formed on the organic EL layer 205.
A glass substrate 207 covers the cathode layer 206. The glass substrate 207 and the glass substrate 200 are attached at their edges, and the R, G, and B organic EL layers 205 are enclosed therein. Here, the R, G, and B organic EL layers 205 are respectively formed by selectively performing vapor-deposition of each of R, G, and B organic EL materials by using a metal mask.
On the other hand, as a method of realizing a full-color organic EL display device without using the above R, G, and B organic EL layers 205, a method of using color filter layers has been proposed. In this case, a combination of a white organic EL layer and color filter layers is employed.
FIG. 6 is a cross-sectional view of such a full-color organic EL display device. An insulating layer 301 as a substrate is formed on the glass substrate 300, and an R color filter layer 302, a G color filter layer 303, and a B color filter layer 304 are formed on the insulating film 301. Each of these color filter layers transmit light having a predetermined wavelength corresponding to each of the R, G, and B colors, which is irradiated from the white organic EL layer. Although not shown, an organic EL element driving TFT is formed under these color filter layers 302, 303, and 304 in a manner similar to the TFT 201 of FIG. 5.
A first planarization insulating film 305 is formed on these color filter layers 302, 303, and 304. Anode layers 306, 307, and 308 are formed on the first planarization insulating film 305, corresponding to each of the R, G, and B colors. A second planarization insulating film 309 is formed so as to cover end portions of the anode layers 306, 307, and 308, and a white organic EL layer 310 and a cathode layer 311 are laminated thereon in this order. Furthermore, a glass substrate 312 covers the cathode layer 311, and the glass substrate 312 and the glass substrate 300 are attached at their edges, so that the white organic EL layer 310 is enclosed therein.
Here, the reason to provide the second planarization insulating film 309 is that the distance between the anode layers 306, 307, and 308 and the cathode layer 311 becomes small without the second planarization insulating film 309 and may cause a short circuit. The second planarization insulating film 309 is formed with openings except above the end portions of the anode layers 306, 307, and 308, and the white organic EL layer 310 is in contact with the anode layers 306, 307, and 308 exposed in the openings.
The organic EL display device of this type is described in a Japanese Patent Application Publication No. Hei 11-251059.
However, the organic EL display device employing the described structure of the white organic EL layer and the color filter layers has following problems. Firstly, since the first planarization insulating film 305 is formed on the R color filter layer 302, the G color filter layer 303, and the B color filter layer 304, a manufacturing cost increases accordingly. This can be solved by eliminating the first planarization insulating film 305 by using the color filter layers as the first planarization insulating film 305. In this case, the adjacent color filter layers need to overlap each other for planarization and for increasing an aperture ratio. However, since the step height at the overlapping portion of the color filter layers becomes large, a display defect may occur by a cut in the organic EL layer, moisture absorption at the step portion, and so on.
Secondly, as the first planarization insulating film 305, organic resin such as acrylic resin having a thickness of 2 to 3 micrometers must be used for planarization. However, since the organic resin has high moisture absorbency, it can have an adverse effect on the organic EL layer which is sensitive to moisture, causing a display defect.